Evidence from remote sensing: Do the transboundary basins exhibit signs of regulated water upstream and associated dangers and stress downstream?

Two major transboundary river systems in the Middle East and the Caucasus, the Tigris-Euphrates and the Kura-Aras basins, originate largely in Türkiye. Although Türkiye constitutes only a limited portion of both basins, it plays a decisive role in shaping water availability across the entire systems. These dynamics have had substantial implications for downstream countries, contributing to severe water stress in the past and potentially intensifying in the future. This issue has therefore been examined using remote sensing data, given the limited availability of ground-based information on water resources and their use in both upstream and downstream basins. Monthly precipitation, temperature vegetation dryness index (TVDI), aridity index, AET/PET, soil moisture, and groundwater storage from 2003 to 2025 were employed for this purpose. The research's findings indicate that while the downstream basins have a distinct approach to water supply and use, the upstream basins follow a similar strategy. In the Kura-Aras, in addition to precipitation and available surface water, a significant portion of the water demands has been supplied by groundwater resources, which show a sharp downward trend in all four areas studied. This has caused the AET/PET index readings to be higher than 0.5 for the majority of the year. Despite a dramatic decline in CRD and soil moisture, AET/PET values in Georgia and Turkiye have not altered much since 2017. However, the rate of groundwater storage has also increased. In contrast, the Tigris-Euphrates, although the trend of groundwater storage decline in the basin’s upper reaches occurs at a much lower slope (Sen’s slope = -2.8), the cumulative rainfall deviation in the majority of years shows a severe deficit, which supports the basin's upstream reaches' regular consumption of surface water resources. However, the AET/PET ratios are less than 0.5 in most months in Iraq, especially in the Southeast, indicating extreme water stress and scarcity. After 2010, although there have been significant swings in the groundwater level, it has consistently followed a straight linear pattern. This could be the result of political issues like a civil war, a lack of infrastructure for exploitation, or a reluctance to use groundwater to make up for the amount of water needed. The findings show that there is a difference in consumption and significant water stress in the Tigris-Euphrates watershed downstream. Although the nations downstream of the Kura–Araks have relied heavily on groundwater resources to meet their water and soil moisture requirements, this could lead to future issues and conflicts.